The present invention relates to a dispersion of ketene dimers for sizing cellulose fibres and to the use of the dispersion. In particular it relates to a dispersion containing a ketene dimer as sizing component and a cationic system of dispersing agents. The properties of the dispersing agent system are such as to make it possible to employ a very large amount of ketene dimer while maintaining the cationic properties and without causing the viscosity of the dispersion to become too high or without decreasing the storage stability of the dispersion.
It is known to use ketene dimers for sizing cellulose fibres. The ketene dimer group reacts with the hydroxy groups of the cellulose molecules resulting in an irreversible bond between the cellulose molecule and the organic, hydrophobic hydrocarbon groups in the ketene dimer molecule. When adding ketene dimers to an aqueous suspension of cellulose fibres, as so called beater additives, the ketene dimers are dispersed in the water by means of dispersing agents.
Moreover, it is known that the yield of the reaction between the ketene dimers and the cellulose fibres increases when the aqueous dispersions of the ketene dimers contain a cationic dispersing agent.
It is believed that the yield of the reaction is improved when using cationic dispersions due to increased affinity of the ketene dimer particles to the slightly negatively charged cellulose fibres.
A serious disadvantage of previously described cationic ketene dimer dispersions is that the amount of ketene dimer in the dispersion must be kept relatively low, usually about 3-8 per cent by weight. This is due partly in order to obtain an optimal cationic activity in relation to the amount of ketene dimer and partly in order not to adversely effect the viscosity to a significant extent along with the manageability and the stability of the dispersions. This, in turn, results in high transport costs of the dispersions from the manufacturer to the consumer and in the necessity of employing relatively large storage equipment.
Experiments to increase the ketene dimer content by introducing additional ketene dimer into a cationic dispersion result, of course, in dispersions having a higher solids content of ketene dimer but alter the cationic activity in the system so that the sizing effect of the dispersion, when being applied to cellulose fibres, was decreased considerably. Increasing the solids content by simultaneously increasing the amount of the ketene dimer and the cationic emulsifier results in an increase in the viscosity of the dispersion to such an extent that the dispersion becomes less manageable. Moreover, at higher concentrations, most of the cationic dispersing agents have a tendency to gelatinize the dispersion at storage.
According to the present invention it has been found that stable dispersions of ketene dimer having a solids content of up to 30 per cent by weight can be obtained, which when diluted and applied onto the cellulose fibres give an improved yield at the sizing reaction compared to most of the previously known dispersions having a lower dry content of ketene dimer.
The dispersing system according to the invention enables the preparation of ketene dimer dispersions having a high solids content without causing the viscosity of the dispersion to become so high that the manageability of the dispersion will decrease. Furthermore these dispersions are, dispite their high solids content, very stable. Storage of the dispersions for 6 months at room temperature does not affect their viscosity.
Another advantage of the dispersions of the present invention is that the amount of dispersing agent based upon the ketene dimer can be kept very low, usually as low as about 4-5 per cent by weight based on the ketene dimer. On the other hand, when using only cationic starch as dispersing agent, it should be present in an amount of at least 30 percent by weight based on ketene dimer, in order to obtain satisfactory dispersions exhibiting good sizing effect. Due to the special dispersing agent system, the compositions according to the present invention do not seem to be sensitive to the oxidizing residual chlorine, such as chlorate ions in the cellulose pulp, which is usually present when the cellulose pulp is bleached with chlorine compounds prior to the paper making.